Gin pennant

Flying the gin pennant was an invitation (generally aimed at officers) to come aboard for a (free) drink in the ship's wardroom.

(Consequently, junior officers would sometimes compete and conspire to fly the gin pennant aboard other ships.)

Gin pennants likely originated in, and remain in use today, in the Commonwealth navies.

Purpose-made gin pennants often take the form of a starboard pennant defaced with a wine, cocktail, or martini glass (typically green, sometimes gray or black) on the field.

Another variant described is a small triangular flag in solid green, defaced with a white cocktail or wine glass.